"Repeating the analogy given above, we should be able to start at the tip of the highest branch of the tree -- us -- and trace backward in a single line, if not to the root (microbes) then to a thicker main branch (this ancestor)."
A straight line going back 2.2 million years from Homo sapiens to Homo erectus to Homo habilis with the common ancestor Australopithecus afarensis at about 2.7 million years (common to Australopithicenes and Homo.
I don't know if they know precisely what differences in DNA sequence and alleles there might have been, because there is no DNA available, to my knowledge.
I don't know why you think the number of mutations between one and the other is that important. It is obvious there are major differences in skeletal structure, especially cranial development.
There is a gap in fossils from 8 mya to 4.4 mya. So we don't have common ancestor information for the gorilla, chimp, human split. But using the molecular clock and DNA-DNA hybridization some conclusions can be drawn.
The difference in amino acid sequences in albumin of chimps, gorillas and humans is only about 1.2%. With this and a simple formula that indicates the relationship between time and distance, a divergence date of about 5MYA can be estimated. Much of the difference between apes and humans are attributable to just a few regulator genes. We are not as genetically different as we look.
I don't know if they know precisely what differences in DNA sequence and alleles there might have been, because there is no DNA available, to my knowledge.
I don't know why you think the number of mutations between one and the other is that important. It is obvious there are major differences in skeletal structure, especially cranial development.
I've explained the importance of this question repeatedly. To get an approximate average for the amount of time between each mutation so I can find out whether the lack of new phenotypes/mutations/species since the modern human appeared (100,000-200,000 years or so) poses a problem for conventional evolutionary theory. Maybe it doesn't. Probably it doesn't. As I said before, averages are just that: averages...
There is a gap in fossils from 8 mya to 4.4 mya. So we don't have common ancestor information for the gorilla, chimp, human split.
In other words, the answer is #3: We don't really know. Now was that so hard? SHEEEESH!
But using the molecular clock and DNA-DNA hybridization some conclusions can be drawn. The difference in amino acid sequences in albumin of chimps, gorillas and humans is only about 1.2%. With this and a simple formula that indicates the relationship between time and distance, a divergence date of about 5MYA can be estimated. Much of the difference between apes and humans are attributable to just a few regulator genes. We are not as genetically different as we look.
Ah, but what a difference that 1.2% makes! If you drink a glass of water that is 1.2% plutonium... The terrorists on the 9/11 planes probably made up less than 1.2% of the total number of persons on the 4 planes. If you attend a big party with 1,000 people, 988 of them say hello or give you some sort of compliment, you'll stay awake wondering about the other 12.
1.2% difference gives us the difference between a creature that can have conversations like this, can expound on moral philosophy, theology, quantum physics, write and be moved by poems, novels, movies, songs -- and a creature that has marginally more capacity for these things than a rock.
(Then there is the question left hanging in the air, implied by the 1.2% number. What to do we have to compare it to? In other words, what percent difference in DNA is there between a man and a rabbit? Or a lizard? I bet those percentages are pretty low too.)
Now please don't be insulted but the thing that disturbs me is that we've gone back and forth about half a dozen times and it took this long to get a semi-straight answer -- and I still had to translate it into "we're missing some DNA from part of that period so we don't really know how many phenotypes there were between this ancestor and us".
I would think that a scientist should be able to organize his thoughts and present them in a fairly clear, concise manner at the drop of a hat instead of this taffy pull. After the short summary you can qualify your answer with more specific details to illuminate the complexities behind the it. We learned this in English class -- paragraphs go from the general to the specific.